This invention relates to an engine rotation control device for controlling the idling speed of an engine.
In order to control operating conditions of an engine such as ignition timing, fuel injection and the like, signals are generally utilized which are generated by a signal generator in synchrony with the rotation of the engine. The signal generator generally senses the rotation of a crankshaft or camshaft operatively coupled therewith. An example of this type of signal generator is schematically illustrated in FIGS. 1 and 2. In FIG. 1, a signal generator in the form of a rotational position sensor is generally designated by referrence numeral 8 and includes a rotating shaft 1 which is rotated in synchrony with the rotation of a multicylinder engine (not shown) which is, in this example, a four-cylinder engine, and a rotary disk 2 secured at its center to the rotating shaft 1 for integral rotation therewith. The rotatry disk 2 has a plurality of windows or slits 3 formed therein around the rotating shaft 1 in a circumferentially spaced relation with respect to each other. Each of the slits 3 corresponds to one of the cylinders of the engine, so for a four-cylinder engine, there are four slits in the disk 2. The slits 3 are equally distant from the center of the rotary disk 2. All the slits 3 have the same length as one another in the circumferential direction of the disk 2. Each of the slits 3 has a leading edge L and a trailing edge T. The leading edges L and the trailing edges T of all four slits 3 are equally spaced around the disk 2 at intervals of 90 degrees.
As shown in FIGS. 1 and 2, a light source 4 in the form of light emitting diode and a light sensor in the form of a phototransistor 5 are disposed in alignment with each other on opposite sides of the rotary disk 2 in such a manner that when one of the slits 3 is aligned with the light emitting diode 4 and the phototransistor 5, light emitted from the light emitting diode 4 can pass through the slot 3 thus aligned and reach the phototransistor 5, which is thereby turned on. At other times, the phototransistor 5 remains off.
In operation, when the light which is generated by the light emitting diode 4 passes through one of the slits 3 in the disk 2 and strikes the phototransistor 5, the phototransistor 5 conducts and current flows through the phototransistor 5 and a resistor 5A which is connected to the emitter of the phototransistor 5. An amplifier 6 amplifies the voltage across the resistor 5A and provides the amplified signal to the base of an open-collector output transistor 7.
FIG. 3 illustrates the output signal of the signal generator 8. The output signal is in the form of pulses having a rising edge corresponding to the leading edge L and a falling edge corresponding to the trailing edge T of each slit 3 in the disk 2. In FIG. 3, a rising edge of an output pulse occurs when the piston position of the corresponding cylinder is at 75.degree. before top dead center (BTDC) whereas the falling edge occurs when the piston position of the corresponding cylinder is at 5.degree. BTDC However, the piston positions corresponding to the rising and falling edges in FIG. 3 are just examples, and different values can be employed.
As shown in FIG. 4 the output signal of the signal generator 8 is inputted to a microcomputer 10 via an interface 9. Based on the output signal from the signal generator 8, the microcomputer 10 controls the ignition timing, the fuel injection, and other aspects of engine operation. For example, in order to stabilize the number of revolutions per minute of the engine, the microcomputer 10 successively determines the intantaneous number of revolutions per minute (rpm) of the engine, for example, by measuring the length of time between the rising or falling edges of two successive pulses of the generator output signal, calculates an average value of the thus determined instantaneous numbers of rpm for a predetermined number of ingitions, compares each instantaneous number of rpm with the corresponding average value so as to obtain a deviation therefrom, and then sends a control signal to make a certain appropriate adjustment or modification of the ignition timing in dependence upon the deviation thus obtained.
With the known engine control device as constructed above, it is possible to reduce instantaneous pulsations or fluctuations in the number of engine rpm, but it is difficult to maintain the number of engine rpm at a prescribed target value. This is because an average value of instantaneous numbers of rpm calulated for a predetermined number of ignitions is fixed and does not always take account of the latest number of rpm most recently sensed or calculated.
Further, the above-described known device involves another problem. Specifically, an adjustment range of engine operation in which appropriate adjustments or modifications in ignition timing are to be made are generally determined based on a threshold value of rpm (e.g., below 1,000 rpm) which is set to be higher than a predetermined target idling number of rpm and a threshold engine load (e.g., an intake vacuum below 400 mm Hg) which is set to be greater than a predetermined target engine load. Accordingly, when the engine is decelerated with a clutch being cut off, such adjustments or modifications are carried out as soon as the operating conditions of the engine get into the adjustment range (e.g., when the number of engine rpm falls below the maximum threshold value of rpm which is greater than the target idling number of rpm). As a result, the engine will often experience an excessive fall or reduction in the number of rpm.